Abstract for presentation at 11th International Congress of Human Genetics

Sex ratio of healthy newborns in Switzerland was 1.04 (male to female: 37340 to 35742) in 2004. Considering the higher prenatal loss rate of males, Huether (1990) extrapolated a sex ratio at conception for whites of approximately 1.15. At least twice as much females than males are born alive with trisomy 18, and females survive longer than males. Huether (1996) reported a prenatal sex ratio of 0.9 for trisomy 18 before and after 16 weeks of gestation and of 0.69 at birth and concluded a prenatal selection against males after 16 weeks.
We collected data on 352 cases with trisomy 18 from 2 cytogenetic laboratories in Zurich between 1964 and May 2003. In 320 cases the week of gestation at the time of cytogenetic diagnosis and the sex were known. For cases diagnosed between 10 and 16 0/7 weeks of gestation, the sex ratio was 1.02 (60 males/59 females), between 16 1/7 and 28 0/7 weeks it was 0.82 (27/33), and between 28 1/7 and 43 weeks it was 0.52 (48/93). From these figures we conclude that (1) there is a continuous higher prenatal loss of male versus female fetuses; (2) that male fetuses have a substantially higher risk for intrauterine demise during the third trimester of gestation and around term, and (3) that the probability of intrauterine death for fetuses diagnosed with trisomy 18 during the first and early second trimester of pregnancy must be given differently for female versus male fetuses. This fact must be considered at genetic counselling when prenatal diagnosis had disclosed fetal trisomy 18.
On the basis of the fetal death rate for trisomy 18 after amniocentesis of about 68% (Hook 1983) and the live-born sex ratio in our study group of 0.56, we calculated a risk of about 60% of fetal demise for female fetuses after 18 weeks of gestation till live-birth, while for male fetuses, this rate would be about 77%.